Schools and Universities use this feature for vacation schedules.
This category option allows you to set events in the category to "Always Display." This means that any event in the category will show regardless of what category or categories selected. This would eliminate duplication of the event.
To set this option:
Login as a calendar administrator -> Go to Admin Menu -> Categories -> Edit or add a category -> Go to "Advanced Properties" tab
Under item 4 "Special Editing and Viewing" check box (E) "Always show this categories events combined with events in selected category." -> Click Save.
Note: If you turn on this setting in a category, the calendar will always display events in this category even if you filter events by the category or location selection lists.